We have discovered a third retinal-containing photoactive protein, sR, which apparently functions as the photoreceptor for the phototaxis of Halobacterium halobium. The pigment closely resembles the other retinyl-proteins previously found in this organism. It has a visible absorption maximum at 587nm and a photointermediate, S373 absorbing at 373nm, which accumulates under physiological illumination conditions. S373 is converted back to the 587nm absorbing state by blue and near UV light. The absorption maxima of the two forms correspond to maxima in the action spectrum for the phototactic responses. We will establish definitively whether or not sR is the receptor for any or all of the phototactic responses in H. halobium and search for other photoactive pigments if it is not. We also propose to isolate sR, to characterize it biochemically and structurally and to analyze its photochemical reactions. We will use appropriate modification of techniques which we have developed and applied successfully to isolate and characterize bacteriorhodopsin (bR) and holorhodopsin (hR). Mutant strains of H. holobium, lacking bR and hR but containing sR, which show full phototactic sensitivity, are available and will facilitate the investigation of the new pigment.